i) Field of the Invention
The invention relates to a piston ring mounted on internal combustion engines such as a car engine.
ii) Description of Related Art
A piston ring for use in an internal combustion engine, particularly in a car engine has heretofore been formed of cast iron, but has been shifted to a so-called steel piston ring made by forming a steel wire into a ring shape. Specifically, the piston ring is generally manufactured by a process having the steps of performing hot working, such as forging and hot-rolling, of an ingot with a predetermined composition to obtain a wire stock, further performing drawing and the like to obtain a steel wire material corresponding to a small sectional shape of the piston ring, and performing hardness tempering and bending to form the ring shape with a determined curvature.
At present it is general to mount three rings on one piston, that is, a top ring, a second ring, and an oil ring from the side of a combustion chamber, but in recent tendency the top and oil rings located in portions exposed severe to severe condition are formed of a steel so that the function is enhanced. As the background thereof, in recent years, the results of researches of an electric car etc. intended to be used instead of an internal combustion engine have become remarkable, and efforts to obtain further improvement of the internal combustion engine have been also made.
Moreover, in recent years, researches are also performed regarding the phenomenon in the interior of an engine, and according to the researches of Saito, et al. (xe2x80x9cResearch on Wear of Diesel Engine under Severe Operation Conditionxe2x80x9d in Research Presentation Lecture Meeting 1999 by Kyushu Branch of the Japan Society of Mechanical Engineers (1999)), it has been pointed out that the second ring formed of cast iron is most worn among the three rings in the current situation. Further reasons for the shifting to the steel ring reside in such respects as the followability needs to be enhanced because of thinning design of a ring structure for enhancing engine environmental performance, and that the mechanical strength of the piston ring accordingly needs to be enhanced, as the background thereof. Furthermore, the technical transfer and expansion easiness regarding the manufacture steps of the piston ring also become the great reasons.
The steel piston ring is greatly superior to the cast iron ring in mechanical property and wear resistance, but is inferior in an anti-seizure property, and this is one of the reasons why the steel piston ring is not used particularly in the second ring. To cope with this problem, as disclosed in JP-A-10-30726, in the steel piston ring a surface treatment such as nitriding etc. is used which is performed on a steel piston ring surface in contact with a cylinder liner. However, the surface treatment is insufficient in the respects of the cost and the prevention of Al adhesion occurring on the contact surface with the piston.
Moreover, there is an attempt to solve the problem without any surface treatment, and JP-B-58-46542 and etc. propose adding of Cr not less than 10% as the composition of the steel piston ring to increase in the steel of the piston ring Cr-based carbides superior in performance/cost. The increasing of the carbide amount can bring about remankable enhancement of wear resistance, however, the enhancing effect of the anti-seizure property is slight, and there occurs such a fear of manufacture trouble as deteriorated machinability.
An object of the present invention is to satisfy all of three respects unachieved by the above-described technique: (1) the anti-seizure property is enhanced all over the surface of the piston ring; (2) a piston ring is manufactured by a material having an enhanced machinability in addition to the enhanced anti-seizure property; and (3) even in the forming into a piston ring shape with a small section, the piston ring can maintain sufficient mechanical properties.
As a result of various studies for solving the above-described problems, the inventors have found that the anti-seizure property and workability are effectively enhanced by adding an adequate amount of S to a low alloy steel containing 0.3 to 0.8% C as the principal constituent and that remarkable effect can be obtained preferably by simultaneously adding S and Ca, and have achieved the present invention.
In conventional technique, S is organized and added to engine oil as an extreme-pressure additive to prevent seizure. On the other hand, the inventors have found that when sulfides such as MnS are made to exist in the steel, S forms in-situ a sulfide film on a newly generated surface occurring on a friction surface by frictional heating, and this effectively enhances the lubricating performance. According to this means, since lubricant substances are distributed in the material, no large amount of lubricating substances needs to be added when raising the lubricating performance in a necessary local place. Moreover, unlike the extreme-pressure additive, no effect disappears even in the case of oil exchange, and a semipermanent function can be expected.
Moreover, the increasing of Cr-based carbides in the steel, the raising of the surface hardness by nitriding, and the like in the conventional technique aim at increasing a difference in wear resistance from the liner material of cast iron under abrasive wear. These also raise the anti-seizure property, however, by promoting the wear of the liner side in the state of uneven contact to thereby increase the contact area thereof, these aim at preventing local, abnormal rise of a specific load from occurring. Namely, these relate to technique for improving the conformability of an initially mounted piston ring, and scarcely contribute to improve the wear resisting property required against durability-deteriorating wear such as adhesion wear.
Furthermore, the excessive enhancement of the wear resistance causes the situation of attacking the liner side, the progress thereof causing the increase of clearance etc., and the blow-by amount related with the exhaust gas amount is increased. Since the above-described effect of S enhances the anti-seizure property by lowering the friction coefficient without promoting the material wear, it brings about such a superior advantage as a state of a small clearance change is maintained even in the progress of the engine operation.
Specifically, according to the first aspect of the invention, there is provided an elongate piston ring material with self-lubrication adapted to be suitable for producing a piston ring for an internal combustion engine, the piston ring material comprising a steel consisting essentially, by mass, of not less than 0.3% but less than 0.8% carbon, 0.1 to 3.0% silicon, 0.1 to 3.0% manganese, 0.03 to 0.3% sulfur, 0.3 to 6.0% chromium, 0 to 3.0% copper, and the balance iron, the piston ring material including sulfide inclusions each having aspect ratio, that is, ratio of maximum size to minimum size, not less than 3 when observed in longitudinal structure of the material, the sulfide inclusions in the longitudinal structure being oriented so that an intersecting angle made between an imaginary, straight line passing the maximum size of any one of the sulfide inclusions and another imaginary, straight line passing the maximum size of another of the sulfide inclusions is not more than 30 degrees.
According to the second aspect of the invention, there is provided a piston ring material with self-lubrication adapted to be suitable for producing a piston ring for an internal combustion engine, the piston ring material comprising a steel consisting essentially, by mass, of not less than 0.3% but less than 0.8% carbon, 0.1 to 3.0% silicon, 0.1 to 3.0% manganese, 0.03 to 0.3% sulfur, 0.3 to 6.0% chromium, 0 to 3.0% copper, and the balance iron, the piston ring material including sulfide inclusions each having aspect ratio, that is, ratio of maximum size to minimum size, not less than 3 when observed in a structure present in a face parallel to an outer periphery of a piston ring formed of the piston ring material, the sulfide inclusions in the structure being provided with such a distribution state as an intersecting angle made between an imaginary, straight line passing the maximum size of any one of the sulfide inclusions and another imaginary, straight line passing the maximum size of another of the sulfide inclusions is not more than 30 degrees. Preferably, not more than 0.01% Ca is added, and Cr is in a range of 3.0 to 6.0%, or 0.3 to 1.0%. Incidentally, the outer periphery of the piston ring material means the outer peripheral surface of the piston ring formed of the material, that is, the surface of the steel wire material which is parallel to the longitudinal direction.
Further, in the piston ring material or in the piston ring, the area ratio of nonmetallic inclusions in the structure may be not more than 2.0%, and in addition to the above-described composition, the piston ring material or the piston ring may contain at least one not more than 0.5% in total selected from the group consisting of V, Nb, and Ti, and/or may contain not more than 1.5% Al.